Rewrite Osc_string not to use the Osc_codec functor

Signed-off-by: John Else <john.else@citrix.com>
Remove the Osc_codec module and change the Osc_transport interface

Signed-off-by: John Else <john.else@citrix.com>
Fix .merlin

Signed-off-by: John Else <john.else@citrix.com>
Implement Unix UDP client/server

Signed-off-by: John Else <john.else@citrix.com>
Implement Lwt UDP client/server

Signed-off-by: John Else <john.else@citrix.com>